Conventionally, in relation to low-molecular organic electroluminescence (hereinafter referred to as OLED) devices, it has been reported that it is possible, by providing a copper phthalocyanine (CuPC) layer as a hole injection layer, to realize enhanced initial characteristics such as a lowered driving voltage, an enhanced luminous efficiency, etc. and, further, an enhanced life characteristic (Non-patent Document 1: Applied Physics Letters, USA, 1996, Vol. 69, pp. 2160-2162).
In addition, it has been reported that it is possible to lower the driving voltage by forming a thin film of a metallic oxide by vacuum evaporation and using the thin film as a hole injection layer (Non-patent Document 2: Journal of Physics D: Applied Physics, UK, 1996, Vol. 29, pp. 2750-2753).
On the other hand, in relation to organic electroluminescence devices using a polymeric luminous material (hereinafter referred to as PLED devices), it has been reported that an effect similar to that in the case of the OLED devices can be obtained by using a thin film comprised of a polyaniline material (Patent Document 1: JP-A 3-273087; Non-patent Document 3: Nature, UK, 1992, Vol. 357, pp. 477-479) or a polythiophene material (Non-patent Document 4: Applied Physics Letters, USA, 1998, Vol. 72, pp. 2660-2662) as a hole transport layer.
In recent years, charge-transporting varnishes have been found out which are each comprised of a uniform solution wherein a highly soluble low-molecular oligoaniline material or oligothiophene material is utilized and is completely dissolved in an organic solvent. Besides, it has been reported that when a hole injection layer obtained from such a varnish is inserted into an organic electroluminescence (hereinafter referred to as organic EL) device, a planarizing effect on an underlying substrate and excellent EL device characteristics can be obtained (Patent Document 2: JP-A 2002-151272; Patent Document 3: WO 2005/043962 pamphlet).
The low-molecular oligomer compounds are low in their own viscosity, so that when they are used with ordinary organic solvents, the process margin in the film forming operation is narrow. Therefore, the low-molecular oligomer compounds are disadvantageous in that it is difficult to form a film with high uniformity in the cases where various coating systems such as spin coating, ink jet coating, spray coating, etc. and/or various baking conditions are used.
In this point, use of various additional solvents makes it possible to regulate the viscosity, boiling point and/or vapor pressure, whereby it has become possible to obtain a formed film surface with high uniformity correspondingly to various coating systems (Patent Document 4: WO 2004/043117 pamphlet; Patent Document 5: WO 2005/107335 pamphlet).
However, at present, with full-scale mass production of organic EL devices being near at hand, there is a demand for further lowering in the driving voltage of the devices.
On the other hand, in recent years, hole injection layers using a metallic oxide have been reviewed. Specifically, it has been reported that a metallic oxide constituting the hole injection layer, upon contact with a hole transport layer, oxidizes the interface to thereby form a doped layer in the hole transport layer, whereby the driving voltage can be lowered (Non-patent Document 5: Applied Physics Letters, USA, 2007, Vol. 91, p. 253504; Non-patent Document 6: Applied Physics Letters, USA, 2008, Vol. 93, p. 043308). However, there is no example of a coating-type material which has an oxidizing property relative to a hole injection material, and, therefore, there is a request for development of a new material.